MDEV-32387 Windows - mtr output on is messed up with large MTR_PARALLEL.

Windows C runtime does not implement line buffering mode for stdio.

This sometimes makes output from different tests interleaved in MTR
MTR relies on this buffering (lines won't  output until "\n") to correctly
work in parallel scenarios.

Implement do-it-yourself line buffering on Windows, to workaround.

mysql-test/lib/mtr_report.pm

@@ -76,6 +76,30 @@ if (-t STDOUT) {
   }
 }
 
+# On Windows, stdio does not support line buffering
+# This can make MTR output from multiple forked processes interleaved, messed up.
+# Below is DYI stdout line buffering.
+my $out_line="";
+
+# Flush buffered line
+sub flush_out {
+    print $out_line;
+    $out_line = "";
+}
+
+# Print to stdout
+sub print_out {
+  if(IS_WIN32PERL) {
+    $out_line .= $_[0];
+    # Flush buffered output on new lines.
+    if (rindex($_[0], "\n") != -1) {
+      flush_out();
+    }
+  } else {
+    print($_[0]);
+  }
+}
+
 sub titlebar_stat($) {
 
   sub time_format($) {
@@ -116,10 +140,10 @@ sub _mtr_report_test_name ($) {
 
   return unless defined $verbose;
 
-  print _name(). _timestamp();
-  printf "%-40s ", $tname;
+  print_out _name(). _timestamp();
+  print_out (sprintf "%-40s ", $tname);
   my $worker = $tinfo->{worker};
-  print "w$worker " if defined $worker;
+  print_out "w$worker " if defined $worker;
 
   return $tname;
 }
@@ -661,14 +685,14 @@ sub mtr_report (@) {
   {
     my @s = split /\[ (\S+) \]/, _name() . "@_\n";
     if (@s > 1) {
-      print $s[0];
+      print_out $s[0];
       &$set_color($s[1]);
-      print "[ $s[1] ]";
+      print_out "[ $s[1] ]";
       &$set_color('reset');
-      print $s[2];
+      print_out $s[2];
       titlebar_stat($s[1]) if $set_titlebar;
     } else {
-      print $s[0];
+      print_out $s[0];
     }
   }
 }
@@ -676,6 +700,7 @@ sub mtr_report (@) {
 
 # Print warning to screen
 sub mtr_warning (@) {
+  flush_out();
   print STDERR _name(). _timestamp().
     "mysql-test-run: WARNING: ". join(" ", @_). "\n";
 }
@@ -683,7 +708,7 @@ sub mtr_warning (@) {
 
 # Print error to screen and then exit
 sub mtr_error (@) {
-  IO::Handle::flush(\*STDOUT) if IS_WINDOWS;
+  flush_out();
   print STDERR _name(). _timestamp().
     "mysql-test-run: *** ERROR: ". join(" ", @_). "\n";
   if (IS_WINDOWS)